I've never seen one of these systems worth even a fifth of what they charge.
In my opnion, unless you know how to assess the value of a domain "on your own" you're in no position to be buying/selling them - you'll get taken for a ride.
THE ONLY thing some do a decent job of is looking for "nasty" word combinations -- such as essexbooks.com, which can cause problems down the road in various filters. Most don't even do this.
I say learn the market a bit (subscribe to some of the RSS feeds and listen to a few podcasts) so you'll have the skills on your own. Also, you can hit a site like
http://www.webconfs.com/ and find a lot of the tools to use for free.